umm.....im speechless nanelle...could be a genetic mutant or maybe a potential new gigundo orange...beats me...did you get any seeds?

looks like pumello/shaddoc on a grafted tree. check the base of the tree and see if the giant fruit aren't comming from a limb that starts low, below a graft.
Also if you can get picturs of the leaves of the 2 different parts that may help identify.
If it is a sucker from below a graft you can live with it if you like the giant fruit, or cut out the limb that has the giant fruit and let the orange have all the vigor of the roots.
Arthur

If it is worth taking a picture of it is worth keeping the branch . The explanation you gave could be correct ,OR It could be a spur mutation . This has been seen in some apples grafts were taken from the trees and new strains were born . I believe STARK has used this system at least once on there giant apples and I think also on their double production apples . Call the Guinness book of world records please .

I think SoftMentor is probably correct. Spur mutations would be minor changes. For an orange to look like that one would be more possible through reproductive hybridization or seed DNA mutation. I would bet that it is a grafted branch. I have grafted 30 types of citruses into one tree to date, it will be more this coming spring.